As said there's so many variables it's hard to get a handle on these things. The only real way to judge your 'pace' isn't a speedo, it's some form of power measurement unfortunately at this time it's an expensive item to buy. PM cranks cost upwards of 4.5x the price of your bike & a power tap hub alone will cost over twice the price of your bike!.
On nice rolling asphalt (such as you find on roads that you really don't want to ride on unless you have to) you'll find that you can go much faster than on a really lumpy unclassified road that's falling to bits. So given the average state of the roads around me I cruise at 19-23mph on the flat in low wind conditions, the actual speed depend on the length of ride, this will result in a ride averaging somewhere between 17 & 20mph in pancake to slightly hilly landscapes. At the higher end of that effort level scale I'll drop most roadies fairly quickly, eg they hold on for about 10-15min before slowly dropping off my rear wheel & some will take the lead for a period. So I'd say that 18mph is a decent cruising speed, on a road bike many people can get up to that kind of pace but you'll find a lot of them can't hold that for more than 20min or so.